The invention relates to an apparatus for producing a saturated salt brine solution. More specifically, the invention relates to an apparatus and method for dissolving salt in an undersaturated brine to bring the solution up to a saturated condition.
Salt brine is used in many industrial processes, either as a dilute solution, or a saturated solution. One of these processes is the production of chlorine by the electrolysis of salt (specifically sodium chloride) brine in diaphragm cells or mercury cathode cells. In most chlorine processes the brine feed for the cells is pumped out of underground wells or salt deposits and treated to remove impurities before being passed into the cells. Saturated brine contains about 320 grams/liter of sodium chloride at ambient temperature and pressure. The concentration of sodium chloride in chlorine cell brines is typically about 10 to 20 grams/liter below the saturation level. The actual salt concentration in the brine depends on various factors, such as mining conditions, conditions for treating the brine, and whether or not other materials are added to the brine between the treating plant and the chlorine cells.
It is desirable to use saturated brine for the feed material to a chlorine cell for several reasons: One reason is that the cell is better able to utilize the power input; another reason is that the yield of the chlorine product is much better from a saturated brine than a solution which is less than saturated. At present, there are two general types of apparatuses useful for increasing the concentration of sodium chloride in brine solution. One of these devices is referred to as upflow saturator (or dissolver), and the other is referred to as a downflow saturator (or dissolver).
In an upflow saturator (bed type) the undersaturated brine and solid salt are introduced into the bottom of an upright saturator tank. The salt forms a bed in the saturator which is fluidized by the upwardly flowing brine, so that the salt dissolves in the brine. If the upward velocity of the brine exceeds about 0.45 ft/min, solid salt is carried over the top of the saturator tank. A saturator of this type, therefore, is undesirable, since it is limited to the fluidizing rate of the solid salt.
A downflow saturator makes use of the principle of gravity. The undersaturated brine enters the top of a tank containing solid salt (usually rock salt). The brine is saturated as it flows downwardly through the salt bed. A major drawback of downflow saturators is that they tend to plug up if fine grain salt is used, such as the type of salt generally available at chloralkali production sites where the brine feed for the chlorine cells is drawn from underground wells.
For example, in downflow saturators where fine grain salt is used, the salt must be supported by a layer of gravel at the bottom of the saturator. Above the gravel layer it is critical to maintain a layer of undisturbed salt at least a foot thick. The solid salt which lies above and in the top layer of gravel, along with very fine, partly dissolved grains which "work" down from above, tend to form a very hard, dense salt cake. In moving through this cake, the brine eventually forms channels in the cake and emerges unsaturated. In addition, extraneous insoluble material may accumulate in the caked mass of salt and gravel. Because of these problems, downflow saturators must be periodically cleaned. If they are left too long without cleaning, the hard salt cake must be broken up, for example, with an air hammer, and then manually removed, along with the gravel.
The use of either of the saturator units described above, in a process for producing chlorine which involves saturating brine from underground wells, would require huge equipment which is not commercially feasible. The upflow brine saturator of this invention eliminates most of the problems associated with saturating weak brine solutions, including the plugging problems of the downflow saturators and the critical nature of the fluidizing rate of the salt in the upflow (bed type) saturators.